It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as "liquid toners" or "liquid developers". A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface.
One particularly useful class of liquid toners has a thermoplastic resin dispersed in a nonpolar liquid. Generally a suitable colorant also is present, such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid, which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0, and a high vapor pressure. The toner particles are less than 10 .mu.m average by area size as determined using the Horiba centrifugal particle size analyzer or less than 30 .mu.m average particle size as determined using a Malvern 3600E Particle Sizer, both described below. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in the nonpolar liquid. The image subsequently may be transferred to a carrier sheet.
Since the formation of proper images depends on the differences of the charge between the liquid developer and the latent electrostatic image, it is desirable to add a charge director compound and preferably adjuvants (e.g., polyhydroxy compounds, aminoalcohols, polybutylene succinimide, and aromatic hydrocarbons) to the liquid developer. Such liquid developers provide images of good resolution, but it has been found that charging and image quality are particularly dependent on the specific pigment that was selected. Some formulations suffer from poor image quality, manifested by low resolution, poor solid area coverage (density), and/or non-uniform coverage. Much research effort has been expended to develop new type charge directors and/or charging adjuvants for electrostatic liquid toners that are not as sensitive to the specific selected pigment.